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Surf Zone Hydrodynamics: Measuring Waves and Currents

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Surf Zone Hydrodynamics: Measuring Waves and Currents ISSN 2047-0371 Surf Zone Hydrodynamics: Measuring Waves and Currents Kris W. Inch1 1 Coastal Processes Research Group, School of Marine Science and Engineering, University of Plymouth ([email protected]) ABSTRACT: Wave breaking is the most significant energy input into wave-dominated coastal environments and is responsible for driving a number of interrelated, hydrodynamic processes in a region known as the surf zone. Surf zone hydrodynamics occur over a range of spatial and temporal scales and are constantly changing. As a result, obtaining measurements of waves and currents in the surf zone can be challenging and field experiments require detailed planning to ensure the collection of useful data. Measurements of surf zone hydrodynamics are divided into two general categories: (1) non-directional wave measurements; and (2) directional wave and current measurements. Near-bed pressure transducers are by far the most common method of measuring the non-directional properties of waves. Acoustic sensors are being used with increasing frequency for Eulerian measurements of surf zone currents, whereas Lagrangian surf zone drifters are most useful for measuring flow patterns in rip currents, surf zone circulation and alongshore currents. This article gives a summary of the various methods of measuring waves and currents in the surf zone. Additionally, simple data analysis techniques to study infragravity waves are demonstrated. KEYWORDS: surf zone, breaking waves, pressure transducer, current meters, infragravity waves. Introduction in determining the breaker type and dissipative characteristics of the surf zone. The surf zone is that part of the shoreface Three types of breaking waves are commonly extending from the seaward boundary of recognised (Figure 1); spilling, plunging and wave breaking to the swash zone; the part of surging (Galvin, 1968). Spilling waves the beach that is alternately covered and typically occur on low gradient beaches and exposed by wave uprush and backwash. dissipate their energy gradually over a wide Breaking waves drive a number of surf zone. With plunging breakers, the interrelated surf zone processes such as the shoreward face of the wave steepens until it creation of turbulent bores, wave set-up, is vertical and the crest curls over and nearshore currents and low frequency plunges forward and downward as an intact motions. Together these processes force the mass of water. Plunging waves are more entrainment and transport of sediment, which energetic than spilling waves at the point of leads to morphological change. For a breaking and are normally associated with comprehensive overview of surf zone narrower surf zones and steeper beaches. processes see Komar (1998), or the Surging waves are found on steep, reflective introductory texts of Aagaard and Masselink beaches where there is often no clear surf (1999), Woodroffe (2002), Davidson-Arnott zone as the waves slide up the beach without (2010) and Masselink et al. (2011) for a more physically breaking. In reality, there is a succinct summary. continuum of breaker types blending from one to another and on a natural beach with a The width and characteristics of the surf zone spectrum of wave heights and periods it is vary constantly, driven by changes in the tide common to see a range of breaker types at a elevation, incident wave height and direction, given time. A number of dimensionless low-frequency motions and local wind speed. parameters have been developed to predict The beach slope also plays an important role British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 2 the breaker type and surf zone state. The break close to shore creating a narrow surf most widely used of these is the Iribarren zone that can remain unsaturated. number � defined as ���� The inner surf zone is of particular � = importance as the conditions here force the �! �! hydro and sediment dynamics in the swash where � is the beach slope, � is the breaker zone, which is arguably the most dynamic ! part of the nearshore region (Masselink and height and �! is the deep water wavelength given by linear wave theory. Spilling waves Puleo, 2006). Water motion in the inner surf zone is typically dominated by infragravity occur when � < 0.4, plunging waves occur waves, particularly on dissipative beaches. when 0.4 < � < 2.0, and surging waves occur Infragravity waves are low-frequency (0.005- when � > 2.0 (Battjes, 1974). 0.05 Hz) waves that approach the shoreline bound to incident wave groups but become free waves in the surf zone following incident wave breaking. Infragravity waves are an important research topic for coastal scientists as they play an important role in beach and dune erosion, especially during storms (Russell, 1993). Due to their long wavelengths, which prevent breaking, infragravity waves reflect from the shoreline and travel seawards giving rise to a cross- shore quasi-standing wave pattern (Guza and Thornton, 1985). Infragravity energy in the surf zone increases with increasing offshore wave height; however, recent studies have revealed that under very energetic wave conditions infragravity waves may also become saturated (e.g. Senechal et al., 2011a; Guedes et al., 2013; De Bakker et al., 2014). In addition to wave motion, three types of quasi-steady, wave-induced currents exist in Figure 1: The three types of breaking wave. the surf zone; bed return flow, alongshore Note the different break point locations in currents and rip currents. These currents relation to the shoreline. Modified from exist simultaneously and are driven by cross- Davidson-Arnott (2010). shore and alongshore gradients in the mean water level caused by variations in the wave In the inner surf zone of low gradient, breaker height. The intensity of nearshore dissipative beaches, the incident wave height currents increases with increasing incident � is controlled by the local water depth wave height. Thus, the strongest currents (Thornton and Guza, 1982) and can be capable of transporting vast quantities of expressed by sediment occur during storms (Senechal et � = � ℎ al., 2011b). Additionally, rip currents pose a significant hazard to water users and are a where � is a coefficient ranging from 0.3-0.6 major cause of lifeguard rescues around the and increases with beach slope, and ℎ is world (Short and Brander, 1999; Scott et al., water depth. When the wave height is limited 2007, 2008). by the local water depth the surf zone is considered to be saturated. Under saturated Measurements of surf zone processes are of conditions, to maintain a constant relationship vital importance for estimating potential storm between wave height and water depth, an damage, modelling shoreline evolution and increase in offshore wave height acts only to designing shoreline management plans. increase the width of the surf zone. However, Waves and currents are well documented in on steep, sandy beaches plunging waves the coastal literature; advances in instrument British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 3 Inch, K.W. technology and analysis techniques have Nowadays, almost all surf zone field improved our knowledge considerably over experiments use bottom-mounted pressure the last few decades. However, certain transducers. Pressure transducers measure aspects remain poorly understood, such as pressure variations associated with passing surf zone hydrodynamics during extreme waves above and this pressure is converted storms. The purpose of this entry is to (either by the sensor or in post-processing) provide an overview of the current in situ into the equivalent water depth. Pressure methods of measuring surf zone waves and transducers are most commonly attached to currents, and to give a simple demonstration a temporary frame driven into the sand of wave data analysis. (Figure 2a), but can also be fixed to permanent features such as shore platforms or buried beneath the surface of the beach. Surf Zone Measurements An advantage of using buried pressure transducers is to avoid corruption of the For convenience, the following section on signal caused by dynamic pressure variations surf zone measurements is split into two main from accelerating and decelerating flows categories: (1) non-directional wave (Austin et al., 2014). Unlike wave staffs, measurement; and (2) directional wave and pressure transducers are not directly current measurement. Non-directional wave exposed to wave action. They are also measurements are measurements of the relatively easy to deploy, can be co-located water surface elevation from which with other sensors and are often self-logging information about wave height and period can eliminating the need for cables. be obtained. Current measurements are measurements of the velocity vector and are The main limitation of using pressure used to study nearshore currents and the transducers is depth attenuation of the directional properties of waves. pressure signal. The extent of depth attenuation is frequency dependant; as depth Non-directional wave measurement increases high frequency signals are lost before low frequency signals. Depth Field measurements of waves have attenuation can be corrected for during post- previously been collected using a form of processing by applying a frequency surface piercing wave staff (among others, dependant depth attenuation factor �(�) Thornton and Guza, 1983; Davidson-Arnott derived from linear wave theory as: and McDonald,
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